1. Field of the Invention
The present invention relates to a data reproducing apparatus and, more particularly, to a data reproducing apparatus suitably adapted for reproducing recorded data from a magnetic tape which is magnetically recorded by a biphase-mark system.
2. Description of the Prior Art
In a video tape recorder (VTR) of, e.g., an 8-mm video format, an audio signal is PCM-encoded and compressed along the time axis. The signal is then modulated by biphase-mark modulation and recorded on an audio track which is formed adjacent and parallel to a video track. Such a VTR is disclosed in U.S. Pat. No. 4,551,771 assigned to the same assignee as this application.
In modulation of the biphase-mark system, as shown in FIG. 9A, frequency signals S.sub.l and S.sub.2 consisting of sine waves, with a ratio of frequencies f.sub.1 and f.sub.2 of 1:2, are recorded as digital data on a magnetic tape while maintaining a phase relation in which zero crossing points coincide with each other. For example, when audo data is in logic state "0", the frequency signal S.sub.1 of low frequency f.sub.1 =2.9 (MHz) is recorded on the magnetic tape, and when the audio signal is in logic state "1", the frequency signal S.sub.2 of high frequency f.sub.2 =5.8 (MHz) is recorded thereon.
In order to reproduce the audio data thus recorded on the audio track, as shown in FIG. 9B, a zero crossing point of the picked-up frequency signal S.sub.1 or S.sub.2 is detected, and a sampling pulse oscillator using, e.g., a PLL (phase locked loop) configuration oscillator produces outputs PLL.sub.O which maintain a phase interval of 45.degree. with respect to the frequency signal S.sub.1 of the low frequency f.sub.1 and that of 90.degree. with respect to the frequency signal S.sub.2 of the high frequency f.sub.2.
Of the pulse outputs PLL.sub.O, pulses generated at phases of 45.degree., 135.degree., 225.degree. and 315.degree. of the frequency signal S.sub.l are used as a sampling pulse Ps (FIG. 9C) to sample a signal level LV.sub.l of the frequency signal S.sub.l, and the signal level LV.sub.l is used as reproduction data of logic state "0".
Of the pulse outputs PLL.sub.o, pulses generated at phases of 90.degree. and 270.degree. of the frequency signal S.sub.2 are used as a sampling pulse Ps to sample a signal level LV.sub.2 of the frequency signal S.sub.2, and the signal level LV.sub.2 is used as reproduciton data of logic state "1".
In a VTR with the 8-mm video forma, a recording signal obtained by time-divisionally multiplexing audio data as shown in FIG. 9A, is generated in a recording information forming circuit 1 during a recording mode and a video signal is recorded on a magnetic tape 4 through a recording amplifier 2 and a pair of rotary magnetic heads 3 and 3', as shown in FIG. 10.
In this case, an audio signal processor in the form of circuit 1 digitally-converts an input analog audio signal, encodes it into a PCM signal, forms a serial data string from the encoded data, and then modulates data "0" as a frequency signal of 2.9 MHz and data "1" as a frequency signal of 5.8 MHz.
The recording information thus recorded on the magnetic tape 4 is picked up by a pair of rotary magnetic heads 11 and 11', and input to an intermediate tuning circuit 13 through a reproducing amplifier 12. Of the reproduced signals, high frequency signal components of a video signal are peaking-corrected, and the obtained video signal VD is supplied to a video signal processor 14. Audio data AD is input to an equalizing circuit 16 of the audio signal processor 15. A switching circuit for selecting the video signal and the audio signal is omitted.
The equalizng circuit 16 is constituted by connecting a low-pass filter 18 having an operational amplifier 17 in series with a high-pass filter 19. An output from the high-pass filter 19 is supplied as an equalizer output ADX to a PCM demodulator 21 through an output amplifier 20, so that an audio signal is demodulated by the PCM demodulator 21. The sampling signal Ps is input to the demodulator 21.
An amplitude characteristic of audio data AD obtained from the intermediate turning circuit 13 is affected by a frequency characteristic of the system from the magnetic tape 4 to the intermediate tuning circuit 13. As described above with reference to FIGS. 9A to 9C, when the frequency signals S.sub.1 and S.sub.2, which are picked up under conditions in which a level difference for clearly distinguishing the signal level LV.sub.l representing logic state "0" from the signal level LV.sub.2 representing logic state "1" at a timing of the sampling pulse Ps is not generated or is degraded, the equalizing circuit 16 corrects the frequency signals S.sub.1 and S.sub.2 using the low- and high-pass filters 18 and 19.
When the level of the picked-up frequency signal S.sub.1 is significantly decreased with respect to that of the frequency signal S.sub.2 as shown in FIG. 11A, the signal level LV.sub.1 of the frequency signal S.sub.l at time tps, at which the sampling pulse Ps is generated, is decreased below the signal level LV.sub.2 of the frequency signal S.sub.2. Accordingly, although the frequency signal S.sub.1 representing logic state "0" is picked up, audio data of logic state "0" cannot be reproduced in accordance with the frequency signal S.sub.1
On the contrary, when a signal level of the frequency signal S.sub.2 is significantly decreased, with respect to the frequency signal S.sub.1, as shown in FIG. 11B, the signal level LV.sub.2 of the frequency signal S.sub.2 at time tps, at which the sampling pulse Ps is generated, becomes extremely low even if the frequency signal S.sub.2 is picked up. Accordingly, the signal S.sub.2 may not be accurately determined as audio data of logic state "1".
Theoretically, when a ratio of the amplitude of the frequency signal S.sub.2 to that of the frequency signal S.sub.1 is about -6 (dB), a level difference between the signal levels LV.sub.1 and LV.sub.2 obtained when the sampling pulse Ps is generated can be practically determined. For this reason, it is assumed that data can be reproduced at a sufficiently low error rate. ln consideration of the above problem, the conventional equalizing circuit 16 equalizes such that the amplitude characteristic of the equalizer output ADX is flattened throughout a range of the low frequency F.sub.1 =2.9 (MHz) to the high frequency f.sub.2 =5.8 (MHz). Frequency characteristics of the low- and high-pass filters 18 and 19 are set to obtain the above characteristic.
However, the frequency characteristic of the system from the magnetic tape 4 to the intermediate tuning circuit 13 is actually different for different types of VTRs. Especially when the types of the magnetic tapes 4 used as recording media are different, influences on the characteristic of the picked-up audio data AD are different from each other. For this reason, equalization cannot be performed correctly by the conventional equalizing circuit 16 of FIG. 10.
For example, as shown in FIG. 12, a coating type magnetic tape 25 formed by coating a magnetic layer 25A obtained by mixing ultrafine particals of an alloy of nickel and cobalt together with a binder into iron and having a thickness of about 3.mu. on a base 25B is often used as a magnetic tape 4. As shown by a curve MP in FIG. 13, the frequency characteristic of the tape is such that a substantially uniform signal level can be obtained in a low frequency range and the signal level is abruptlly decreased toward a high frequency range, and an amplitude decrease of about .DELTA.G.sub.12 =2 to 3 (dB) occurs near 5.8 (MHz).
On the contrary, as shown in FIG. 14, when a vapor metal deposition type magnetic tape 26, obtained by forming a 0.15-.mu. thick magnetic layer 26A on a base 26B by sputtering, is used, signal components of a low frequency range pass through the magnetic layer 26A during recording since the magnetic layer 26A is thin. Accordingly, as shown by a curve ME in FIG. 13, the trailing characteristic appears in a low frequency range, and the signal level near 2.9 (MHz) is decreased by about .DELTA.G.sub.11 =1 (dB). On the contrary, in the high frequency range, the characteristic extends toward a high range than with the coating type magnetic tape 25, and the amplitude is not decreased near f.sub.2 =5.8 (MHz).
As described above, two types of the magnetic tapes 25 and 26 having different frequency characteristics are sometimes used as a magnetic tape 4 of a VTR. In this case, if the conventional equalizing circuit 16 is used, it is very difficult to obtain the equalizer output ADX by which a ratio of signal levels of the frequency signals S.sub.1 and S.sub.2 becomes an optimal value (i.e., -6 (dB)) for both the coating type and vapor deposition type magnetic tapes 25 and 26.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of the above situation, and has as its object to provide a data reproducing apparatus in which, when a frequency characteristic of reproduced data picked up from a video head 11 changes in accordance with different types of magnetic tape, an equalization characteristic of an equalizing circuit can be adjusted to an optimal value in accordance therewith.
When the type of magnetic tape is changed, the amplitude characteristic and the phase characteristec of the reproducing transducer head output are changed. Therefore, tne eye-opening ratio of an eye-pattern of the head output is decreased, and a bit error rate of demodulated data may be increased.
When the characteristic of an equalizing circuit is changed, an equalization characteristic is switched in accordance with the characteristic change. Therefore, equalization processing is performed such that the eye-opening ratio of the eye-pattern of the equalizer output is maximized, thereby decreasing the bit error rate of the reproduced data .